In general, a ship propelling apparatus includes one spiral propeller. However, in the case of a propelling apparatus including one propeller, the rotational energy of water streams generated when the propeller rotates cannot be used as a propulsive force, thereby causing a high energy loss.
A counter-rotating propeller (CRP) is capable of collecting rotational energy, which may be lost, as a propulsive force. In the CRP, a propulsive force is generated as two coaxial propellers rotate in opposite directions. The rotational energy of a fluid passing through a front propeller is collected as a propulsive force by a rear propeller as the rear propeller rotates reversely. Thus, the CRP exhibits a higher propelling performance than a propelling apparatus including one propeller.
However, since the CRP includes a counter-rotating device enabling two propellers to rotate in opposite directions, a hollow shaft, etc., the CRP is thus relatively difficult to manufacture, install, maintain, and repair.
US Patent Publication No. US2011/0033296 (publication date: Feb. 10, 2011) and Japanese Patent Application Publication No. sho 62-279189 (publication date: Dec. 4, 1987) have disclosed examples of the CRP described above. US Patent Publication No. US2011/0033296 has disclosed a CRP including a planetary gear type counter-rotating device and a hollow shaft installed in a hull of a ship. Japanese Patent Application Publication No. sho 62-279189 has disclosed a double counter-rotating apparatus which is a planetary gear type counter-rotating apparatus installed in the tail of a ship.